Is the gravity gun the quintessential videogame tool/weapon/keepsake? After Gordon Freeman popularised it all those years ago, loads of games and mods have served up their own interpretations, and it's even taken centre stage in a nifty Half Live live-action performance. 2D sidescrolling physics puzzler Nebula [Steam page] is the latest to call upon the gravity gun's services and having spent a bit of time…